Trans-acting factors in yeast pre-rRNA and pre-snoRNA processing

The major intermediates in the pathway of pre-rRNA processing in yeast and other eukaryotes were originally identified by biochemical analyses. However, as a result of the analysis of the effects of mutations in trans-acting factors, the yeast pre-rRNA processing pathway is now characterized in far more detail than that of other eukaryotes. These analyses have led to the identification of processing sites and intermediates that were either too close in size or too short lived to detected by biochemical analyses alone. In addition, it was generally unclear whether pre-rRNA processing steps were endonucleolytic or exonucleolytic; analyses of trans-acting factors is now revealing a complex mixture of endonucleolytic and exonucleolytic processing steps. Many of the small nucleolar RNAs (snoRNAs) are excised from larger precursors. Analyses of trans-acting factors are also revealing details of pre-snoRNA processing in yeast. Interestingly, factors involved in pre-snoRNA processing turn out to be components that also function in pre-rRNA processing, suggesting a potential mechanism for the coregulation of rRNA and snoRNA synthesis. In general, very little is known about the regulation of pre-rRNA processing steps. The best candidate for a system regulating specific pre-rRNA processing reactions has recently been revealed by the analysis of a yeast pre-RNA methylase. Here we will review recent data on the trans-acting factors involved in yeast ribosome synthesis and discuss how these analyses have contributed to our current view of this complex process.